Conventional cryogenic air separation processes that produce oxygen or oxygen and argon with nitrogen are commonly based on a dual pressure cycle. Air is first compressed and subsequently cooled by counter-current heat exchange with warming product streams. The cooled and compressed air is introduced into two fractionating zones, the first of which is at a pressure on the same order as that of the air. The first fractionating zone is thermally linked with a second fractionating zone which is at a lower pressure. The two zones are thermally linked such that a condenser of the first zone reboils the second zone. The air undergoes a partial separation in the first zone producing a substantially pure liquid nitrogen fraction and a liquid fraction enriched in oxygen.
The enriched oxygen fraction is an intermediate feed to the second fractionating zone. The substantially pure liquid nitrogen from the first fractionating zone is used as reflux at the top of the second fractionating zone. In this fractionating zone the separation is completed, producing substantially pure oxygen from the bottom of the zone and substantially pure nitrogen from the top.
When argon is produced in the conventional process, a third fractionating zone is employed. The feed to this zone is a vapor fraction enriched in argon which is withdrawn from an intermediate point in the second fractionating zone. The pressure of this third zone is of the same order as that of the second zone. In the third fractionating zone, the feed is rectified into an argon rich stream which is withdrawn from the top, and a liquid stream which is withdrawn from the bottom of the third fractionating zone and introduced to the second fractionating zone at an intermediate point.
Reflux for the third fractionating zone is provided by a condenser which is located at the top. In this condenser, argon enriched vapor is condensed by heat exchange from another stream, which is typically the enriched oxygen fraction from the first fractionating zone. The enriched oxygen stream then enters the second fractionating zone in a partially vaporized state at an intermediate point, above the point where the feed to third fractionating zone is withdrawn.
The separation of air, a ternary mixture, into nitrogen, argon and oxygen may be viewed as two binary separations. One binary separation is the separation of the high boiling point oxygen from the intermediate boiling point argon. The other binary separation is the separation of the intermediate boiling point argon from the low boiling point nitrogen. Of these two binary separations, the former is more difficult, requiring more reflux and/or theoretical trays than the latter. Argon-oxygen separation is the primary function of third fractionating zone and the bottom section of the second fractionating zone below the point where the feed to the third zone is withdrawn. Nitrogen-argon separation is the primary function of the upper section of the second fractionating zone above the point where the feed to the third fractionating zone is withdrawn.
The ease of separation is also a function of pressure. Both binary separations become more difficult at higher pressure. This fact dictates that for the conventional arrangement the optimal operating pressure of the second and third fractionating zones is at or near the minimal pressure of one atmosphere. For the conventional arrangement, product recoveries decrease substantially as the operating pressure is increased above one atmosphere mainly due to the increasing difficulty of the argon-oxygen separation.
There are other considerations, however, which make elevated pressure processing attractive. Distillation column diameters and heat exchanger cross sectional areas can be decreased due to increased vapor density. Elevated pressure products can provide substantial compression equipment capital cost savings.
In some cases, integration of the air separation process with a power generating gas turbine is desired. In these cases, elevated pressure operation of the air separation process is required. The air feed to the first fractionating zone is at an elevated pressure of approximately 10 to 20 atmospheres absolute. This causes the operating pressure of the second and third factionating zones to be approximately 3 to 6 atmospheres absolute. Operation of the conventional arrangement at these pressures results in very poor product recoveries due to the previously described effect of pressure on the ease of separation.
Accordingly, it is an object of this invention to provide a cryogenic rectification system which can produce nitrogen, oxygen and argon product by the cryogenic rectification of feed air with high product recoveries even at elevated pressure operation.